Permeable or semipermeable membranes are known to be used in a variety of fluid separations including reverse osmosis and gas separations. These membranes can offer, inter alia, significant cost savings over other fluid separation means, such as adsorption and distillation means. The effectiveness of the membranes in fluid separation, however, is highly dependent on the permeability and selectivity of the membranes, which, in turn, are often dependent on membrane forming materials.
Sulfonated aromatic polymers are known in the art as materials useful in the preparation of fluid separation membranes. Among sulfonated aromatic polymers, those that can be used advantageously to prepare fluid separation membranes, including gas separation membranes, are sulfonated polysulfones, sulfonated poly(phenylene oxides), sulfonated polyetherketones, sulfonated polyamides and sulfonated polyureas to name a few. Examples of preparation of sulfonated aromatic polymer-based membranes can be found in U.S. Pat. Nos. 4,971,695; 4,954,143; 4,866,099; 4,717,395; 3,780,496; 3,735,559; 3,709,841, and European Patent Application publication 0394505.
Preparation of asymmetric and composite membranes from sulfonated aromatic polymers is described extensively in the art. For example, preparation of asymmetric polysulfone membranes is described in U.S. Pat. No. 3,855,122, and by Graefe et al. in "Research on Advanced Membranes for Reverse Osmosis," an annual report to the Office of Saline Water, Department of the Interior, Contract No. 14-30-2999, 1973. Preparation of composite sulfonated polysulfone membranes is described in U.S. Pat. Nos. 5,009,678 and 4,981,498, and by J. E. Cadotte et al. in "Research on In Situ-Formed Condensation Polymer for Reverse Osmosis Membranes," Final Report, Office of Water Research and Technology, U.S. Department of the Interior, Contract No. 14-34-001-6521, 1978. Preparation of composite sulfonated poly(phenylene oxide) membranes is described by A. F. Graefe in "Development of a Composite Reverse Osmosis Membrane for Single Pass Seawater Desalination," Final Report to the Office of Water Research and Technology, Contract No. 14-34-0001-7541, 1979, and by R. Huang and J. Kim in the Journal of Applied Polymer Science, Volume 29, page 4029, 1984.
The composite membranes are frequently prepared from an acid form of the respective sulfonated aromatic polymer due to the improved solubility of acid form in solvents convenient in the preparation of composite membranes, in particular, alcohols. However, the use of salt forms of sulfonated aromatic polymers is frequently preferred due to the improved stability of salt forms, as described in U.S. Pat. Nos. 3,875,096 and 3,780,496.
Typically, the composite membranes comprised of sulfonated aromatic polymer are prepared by dissolving the sulfonated aromatic polymer in acid form in a solvent to form a coating solution and then applying the coating solution to the surface of a porous substrate. The acid form of the sulfonated aromatic polymer is utilized to form the coating solution because of its advantageous solubility characteristics and its ability to prepare composite membranes from inexpensive common solvents, such as alcohols and their mixtures with water. After coating, the acid form of the sulfonated aromatic polymer is usually converted to the stable salt form by exchanging with a salt to reduce degradation of the resulting composite membranes during fluid separations. This exchange step not only represents an additional manufacturing step, but also can cause defects in composite gas separation membranes. The defects can be formed during the exchange step as a result of excessive swelling of the separation layer and are particularly deleterious to gas separation membrane performance.
There have been attempts to prepare composite membranes by coating a porous substrate directly with salified sulfonated aromatic polymers. The use of salt forms of sulfonated aromatic polymers to form composite membranes is described in U.S. Pat. 4,818,387 and in European Patent Applications having publication numbers 0277834 and 0404416. However, these membranes were formed from aggressive and/or toxic solvents, such as methoxyethanol and formic acid. To improve membrane permeation rates of composite membranes in reverse osmosis and ultrafiltration applications, additives are frequently included into coating formulations. Typical additives include polyhydric alcohols, such as glycerine; salts, such as lithium chloride; or organic acids, such as citric or lactic acid.
The composite membranes of prior art produced from salt forms of sulfonated aromatic polymers are frequently prone to defects and/or exhibit reduced permeation rates, in particular, as composite gas separation membranes. These low permeation rates result from the use of strong solvent systems required to dissolve the salt forms of sulfonated aromatic polymers. Strong solvent systems will frequently adversely affect the porous support structure, in particular, the polysulfone support structure which is frequently preferred in the preparation of composite membranes.
Therefore, there is a genuine need in the art for composite membranes having enhanced permeation and separation characteristics, which are not prone to defects and/or which does not exhibit reduced permeation rate during liquid or gas separation.